U.S. patent number 5,038,398 [Application Number 07/391,200] was granted by the patent office on 1991-08-06 for method of assigning communication links in a dynamic communication network.
This patent grant is currently assigned to Harris Corporation. Invention is credited to M. Scott Wills.
United States Patent |
5,038,398 |
Wills |
August 6, 1991 |
Method of assigning communication links in a dynamic communication
network
Abstract
A method of assigning communicaton links in a dynamic
communication network, such as a network of earth orbiting
satellites arranged, for example in three constellations. The
assignments are made in phases, with the first phases assigning
high priority link classes. Subsequent phases assign lower priority
link classes. A set of constraints is established, and a high
priority subset of these constraints can be assured by requiring
that any assignment set to be accepted results in a network
topology satisfying that subset of constraints even if existing
links must be intentionally broken. Other constraints are met only
if existing links need not be broken. To assign the links, the set
of candidate nodes having unassigned transceivers is determined,
then a set of possible links that can be formed by the set of
candidate nodes is determined, the possible links are evaluated to
determine a set of link assignments satsifying the greatest number
of the high priority constraints, and the assigned links are
established.
Inventors: |
Wills; M. Scott (Melbourne,
FL) |
Assignee: |
Harris Corporation (Melbourne,
FL)
|
Family
ID: |
23545678 |
Appl.
No.: |
07/391,200 |
Filed: |
August 9, 1989 |
Current U.S.
Class: |
455/13.1;
342/358; 455/509; 455/17 |
Current CPC
Class: |
H04B
7/2123 (20130101) |
Current International
Class: |
H04B
7/212 (20060101); H04B 007/185 () |
Field of
Search: |
;455/3-5,8-9,12-13,16-17,166,179,186,33,63,67 ;342/352,356,358 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Eisenzopf; Reinhard J.
Assistant Examiner: Faile; Andrew
Attorney, Agent or Firm: Antonelli, Terry, Stout &
Kraus
Government Interests
This invention was made with United States Government Support under
Contract No. F 30602-86-C-0224. The United States Government has
certain rights in this invention.
Claims
What is claimed is:
1. In a communication network including a plurality of
communication nodes, each node having at least one communication
transceiver adapted to form a communication link with a compatible
communication transceiver at another of said nodes, a method of
assigning communication links between transceivers at the nodes,
said method comprising at each node the steps of:
(a) storing a set of link assignment constraints;
(b) designating a subset of the constraints as high priority
constraints;
(c) determining a set C of candidate nodes having an unassigned
transceiver with which an unassigned transceiver at said each node
is compatible to form a communication link;
(d) determining a set L of possible links that satisfy the high
priority subset of constraints and that can be formed by said
unassigned transceiver at said each node and the unassigned
transceivers at the candidate nodes in set C;
(e) evaluating the possible links in set L to determine a set of
link assignments in set L satisfying the greatest number of the
stored set of constraints; and
(f) assigning the links in accordance with the set of link
assignments determined in step (e).
2. A method as claimed in claim 1, wherein the communication links
include links within a plurality of communication link classes
having different communication characteristics, said method further
comprising performing steps (c) through (f) once for each
class.
3. A method as claimed in claim 2, further comprising assigning
selected transceivers for communication links of a selected one of
the classes, assigning the selected class of links priority over
links of other classes, and designating as one of the high priority
constraints the assigning of all of the selected transceivers to
links of the selected class, whereby if a selected transceiver is
unassigned following the performing of step (d) for its class of
links, a link of another class is broken, the transceivers from the
broken link are added to set C, and steps (c) and (d) are repeated
for that class of links.
4. A method as claimed in claim 1, wherein the communication nodes
are earth orbiting communication satellites.
5. A method as claimed in claim 4, wherein step (d) includes
determining whether the pairs of nodes forming each possible link
are in line of sight with each other.
6. A method as claimed in claim 5, wherein step (d) further
includes determining whether the transceivers forming each possible
link are compatible to form a communication link.
7. A method as claimed in claim 4, wherein step (d) includes
determining whether the transceivers forming each possible link are
compatible to form a communication link.
8. A method as claimed in claim 4, wherein step (e) includes
determining the set of links formed by the set of satellite pairs
having the maximum expected time in line of sight.
9. A method as claimed in claim 8, wherein the set of satellite
pairs having the maximum expected time in line of sight is
determined by direct computation, assuming a spherical earth.
10. A method as claimed in claim 8, wherein the set of satellite
pairs having the maximum expected time in line of sight is
determined at time t for those satellite pairs having a common
finite T.sub.max by determining the pairs of satellites that are
closest together at time ##EQU2## where for each satellite pair
T.sub.max is the maximum time over which the pair can remain in
line of sight with each other.
11. A method as claimed in claim 1, wherein step (d) includes
determining whether the transceivers forming each possible link are
compatible to form a communication link.
12. A method as claimed in claim 1, wherein step (e) includes
determining whether the set L includes a number of links sufficient
for desired communication in the network, and if not, determining
at least one existing link to be broken to provide additional
unassigned transceivers and repeating steps (c) through (e).
13. A method as claimed in claim 1, further comprising:
at each node at which a change of a link from the node occurs,
broadcasting, to all the communication nodes with which said each
node is forming a communication link, an update of the network
topology as a result of the change, and repeating steps (c) through
(f); and
at each node at which a broadcast of an update of the network
topology is received, repeating steps (c) through (f).
14. A method as claimed in claim 13, wherein following the
broadcast of a first update of the network topology, each node
waits a first preset time interval before repeating steps (c)
through (f) to permit broadcasting of additional updates of the
network topology.
15. A method as claimed in claim 14, further comprising repeating
steps (c) through (f) at second preset time intervals.
16. A method as claimed in claim 14, wherein the first preset time
interval is of a duration sufficient to permit receiving of any
additional broadcasts of updates of the network topology resulting
from the same topology change that resulted in the broadcast of the
first update.
17. A method as claimed in claim 1, further comprising repeating
steps (c) through (f) at preset time intervals.
18. In a communication network including a plurality of
communication nodes, each node having at least one communication
transceiver adapted to form a communication link with a compatible
communication transceiver at another of said nodes, an apparatus
for assigning communication links between transceivers at the
nodes, said apparatus comprising at each node:
means for storing a set of link assignment constraints;
means for designating a subset of the constraints as high priority
constraints;
first determining means for determining a set C of candidate nodes
having an unassigned transceiver with which an unassigned
transceiver at said each node is compatible to form a communication
link;
second determining means for determining a set L of possible links
that satisfy the high priority subset of constraints and that can
be formed by said unassigned transceiver at said each node and the
unassigned transceivers at the candidate nodes in set C;
evaluating means for evaluating the possible links in set L to
determine a set of link assignments in set L satisfying the
greatest number of the set of constraints stored in said storing
means; and
assigning means for assigning the links in accordance with the set
of link assignments determined by said evaluating means.
19. Apparatus as claimed in claim 18, wherein said transceivers
include transceivers capable of forming communication links of a
plurality of communication link classes having different
communication characteristics, said apparatus further comprising
means for storing the assignments of selected transceivers for
communication links of a selected one of the classes; means for
storing the assignment of links to the selected class, the assigned
links having priority over links of other classes; means for
designating as one of the high priority constraints the assigning
of all of the selected transceivers to links of the selected class;
and means responsive to a selected transceiver remaining unassigned
following the determining of the set L for breaking a link of
another class to permit the transceivers from that link to be
utilized with the unassigned selected transceivers to form a link
of the selected class.
20. Apparatus as claimed in claim 18, wherein the communication
nodes are earth orbiting communication satellites.
21. Apparatus as claimed in claim 20, wherein the second
determining means includes means for determining whether the pairs
of noes forming each possible link are in line of sight with each
other.
22. Apparatus as claimed in claim 21, wherein the second
determining means further includes means for determining whether
the transceivers forming each possible link are compatible to form
a communication link.
23. Apparatus as claimed in claim 20, wherein the second
determining means includes means for determining whether the
transceivers forming each possible link are compatible to form a
communication link.
24. Apparatus as claimed in claim 20, wherein the evaluating means
includes means for determining the set of links formed by the set
of satellite pairs having the maximum expected time in line of
sight.
25. Apparatus as claimed in claim 24, wherein the means for
determining the set of satellite pairs having the maximum time in
line of sight comprises means for directly computing the expected
times in line of sight for the links of set L, assuming a spherical
earth.
26. Apparatus as claimed in claim 24, wherein the means for
determining the set of satellite pairs having the maximum expected
time in line of sight comprises means for determining T.sub.max for
the links of set L, and means for determining at time t the pairs
of satellites that are closest together at time ##EQU3## for those
satellite pairs having a common finite T.sub.max, where for each
satellite pair T.sub.max is the maximum time over which the pair
can remain in line of sight with each other.
27. Apparatus as claimed in claim 18, wherein the second
determining means includes means for determining whether the
transceivers forming each possible link are compatible to form a
communication link.
28. Apparatus as claimed in claim 18, wherein the evaluating means
includes means for determining whether the set L includes a number
of links sufficient for desired communication in the network, and
means responsive to a determination that the set L does not include
a sufficient number of links for determining existing links to be
broken to provide additional unassigned transceivers.
29. Apparatus as claimed in claim 18, further comprising:
means responsive to a change of a link from the node for
broadcasting, to all the communication nodes with which said each
node is forming a communication link, an update of the network
topology as a result of the change; and
means responsive to receipt of a broadcast of an update of the
network topology for activating said first determining means, said
second determining means, said evaluating means, and said assigning
means to provide an updated set of link assignments.
30. Apparatus as claimed in claim 29, wherein said responsive means
includes a first timer for timing a preset time interval before
activating, to permit broadcasting of additional updates of the
network topology.
31. Apparatus as claimed in claim 30, further comprising a second
timer for timing a second preset time interval; and means
responsive to completion of timing of the second time interval for
activating said first determining means, said second determining
means, said evaluating means, and said assigning means to provide a
new set of link assignments.
32. Apparatus as claimed in claim 30, wherein the first preset time
interval is of a duration sufficient to permit receiving of any
additional broadcasts of updates of the network topology resulting
from the same topology change that resulting in activation of the
first timer.
33. Apparatus as claimed in claim 29, further comprising a timer
for timing a preset time interval; and means responsive to
completion of timing of the time interval for activating said first
determining means, said second determining means, said evaluating
means, and said assigning means to provide a new set of link
assignments.
Description
BACKGROUND OF THE INVENTION
The present invention pertains to a technique for adaptively
assigning communication links between communication nodes in a
highly dynamic communication network such as a space-based network.
The topology of such a network changes frequently due to various
factors. By way of example, in a satellite network, satellite pairs
are continuously going into and out of line-of-sight with each
other, due to relative motion between the two satellites,
particularly where the network is made up of two or more
constellations of satellites at different orbital altitudes. As a
consequence, many satellite pairs are continuously acquiring and
losing the capability for direct communication between the two
satellites of the pair. Furthermore, in hostile environments,
threats from, for example, directed energy weapons, antisatellite
weapons and jammers alter network topology. Satellites designed to
survive such scenarios must carefully allocate their communication
resources to maintain network performance. Each communication node
in the network must have the ability to intelligently evaluate the
existing network topology and mission-dependent criteria and to
assign available resources such as transceivers, antennae, power,
and bandwidth so as to maximize utilization and effectiveness
Maintaining optimum communications further requires that the
communication nodes be able to drop some links to establish more
favorable ones and to take part in a coordinated search for
neighbor nodes which are known to be isolated from the network.
To date, the bulk of satellite communication applications have
revolved around space-to-ground links where human intervention is
available to aid decision making and control. Entirely space-based
networks require a high degree of reliability and the capability of
autonomous operation.
SUMMARY OF THE INVENTION
The present invention is a process for dynamicly assigning
communication links in a communication network in a way which
exploits the inherent structure of the network topology. The
process has been reduced to practice in a high fidelity distributed
network simulation. Other proposed processes for dynamic link
assignment produce irregular topologies and are generally incapable
of managing a variety of transceiver types which are not compatible
with one another. Most of these processes are untested in a
distributed environment, particularly an environment like that of a
space-based communication network. The present invention is more
suited to the reliability and testability requirements of such
networks. Simulation of the process has allowed evaluation of the
process's performance in the context of a packet level discrete
event simulation with distributed database management, where
network management functions are decentralized among the various
nodes, for example individual platforms in a satellite network.
The algorithm which forms a part of the present invention is
organized into a set of phases in which different classes of links
are assigned according to a set of mission oriented constraints.
The phases are selected according to known mission requirements to
decouple as much of the assignment process as is feasible and to
give priority to certain classes of communication links. The
assignment selection criteria are formulated as a set of
constraints that are methodically applied to reduce the number of
possible assignments under consideration. When this reduction
process is complete, a unique solution may still not be apparent,
but finding the solution is reduced to a manageable task. At this
point a guided search is applied to arrive at the best solution.
All nodes in the network exchange link status information and
compute the algorithm in response to certain stimuli. Thus, all
nodes come to a consistent decision in an implicitly coordinated
manner.
A communication network may have more than one type of
transceivers, for example RF transceivers and laser transceivers
resulting in different types or classes of links. Further, some
classes of links may have a higher priority than other classes, for
example links between nodes in separate sub-networks may have
priority over links between nodes within the same sub-network. Each
class of links is assigned in a separate phase in a prioritized
order. By way of example, a typical satellite network may have
three constellations or sub-networks of satellites which have three
different altitudes, such as an outer layer of high earth orbiting
satellites, an inner layer of low earth orbiting satellites, and a
middle layer of intermediate altitude earth orbiting satellites,
with the outer layer in a geosynchronous orbit. In this example,
the assignment phases of the algorithm may be:
1. Assignment of links between constellations.
2. Assignment of links within one or more of the
constellations.
Phase 2, during which intraconstellation links are assigned, can be
undertaken as many times as there are constellations. However, one
or more of the constellations may be without intraconstellation
links.
Communication requirements often dictate that links between
constellations are of higher value than links within a
constellation. Assignment of these two types of links are highly
interdependent, since assignment of a transceiver to an
intraconstellation communication link precludes its use in an
inter-constellation link. The algorithm of the present invention
can deal with this requirement by allocating the first phase to
assignment of inter-constellation links. If necessary,
intraconstellation links will be disconnected to establish the best
possible set of inter-constellation links.
Later phases of the algorithm are devoted to assignment of
intraconstellation links Since there would be no conflicts between
constellations, all intraconstellation link assignment phases can
be undertaken simultaneously. Separating the algorithm into such
phases has the advantage of considering decoupled segments of the
problem separately, reducing computational complexity via a
divide-and-conquer technique. Execution of the algorithm control
structure is repeated for each phase.
A set of constraints is formulated and applied in a prioritized
manner to limit the number of acceptable assignment sets to a
manageable size. The following set of constraints, formulated for a
large communication network architecture, has been employed with
good results:
1. The limited number of transceivers reserved for links in high
priority classes must be fully utilized.
2. The total number of assigned links in the network must be
maximized.
3. The expected duration of the resulting network topology must be
maximized.
4. The total length of all links in the network must be kept to a
minimum.
These constraints are listed in order of importance. High priority
can be given to some of the constraints in the various phases by
requiring that those constraints must be met by any assignment
selected in that phase, if possible, and allowing existing links to
be broken to establish new links which meet these criteria. As an
example, in a three constellation satellite network, if utilization
of all transceivers reserved for interconstellation links is the
only high priority constraint, this implies that should any
transceiver reserved for interconstellation links (phase 1) remain
unassigned at the end of the corresponding assignment phase, a link
will be broken at the best potential candidate, selected according
to the above criteria, and phase 1 of the algorithm will then be
recomputed, considering the nodes so liberated.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other aspects and advantages of the present invention are
more apparent from the following detailed description and claims,
particularly when considered with the accompanying drawings. In the
drawings:
FIG. 1 is a schematic representation of a communication network
incorporating the present invention;
FIG. 2 presents a data flow diagram of the algorithm for use in a
communication network in accordance with the present invention;
FIG. 3 is an Ada package specification for the algorithm of FIG.
2;
FIGS. 4A and 4B, when positioned as depicted in FIG. 4C, comprise
FIG. 4 which presents a top level Ada pseudo code description of
the algorithm of FIG. 2;
FIGS. 5-7 present a top level Ada pseudo code description of
procedures within FIG. 4; and
FIG. 8 is a block diagram representation of a communication node;
and
FIG. 9 presents the topology database entry for each node in
accordance with the present invention, as defined in this Ada
pseudo code.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
The present invention will be described with reference to a
baseline concept description (BCD) of a representative
communication network having three constellations of earth orbiting
satellites, as illustrated in FIG. 1, including an outer
constellation 10 of boost surveillance and tracking satellites
(BSTS), an intermediate constellation 12 of space surveillance and
tracking satellites (SSTS), and an inner layer 14 of carrier
vehicles (CV). FIG. 2 presents a data flow diagram of the algorithm
utilized in such a communication network in accordance with the
present invention. The interface with the algorithm is described in
the Ada package specification set forth in FIG. 3.
With reference to FIG. 3, the Link--Class type enumerates the
different assignment phases which are carried out by the algorithm.
The Classify--Link function is used to categorize prospective links
into the appropriate one of the assignment phases and is accessible
by other network management entities. The
Schedule--Link--Assignment procedure sets a timer which triggers
the next link assignment computation. This computation occurs at
least once per minute, but no more frequently than once per second.
Finally, the Assign--Links procedure actually computes the
algorithm and returns the Assignments for the node designated by
the parameter --My Address."
The result of Assign--Links is given in the Assignment array, which
contains one entry per transceiver at node My-Address. Each entry
is either the address of the node to which the corresponding
transceiver is assigned, or "Null" if that transceiver is to remain
unassigned. This information is compared with the current link
status at node My--Address to determine the required actions If a
currently active transceiver receives a null assignment, its link
is broken and it is not reassigned. If it receives a non-null
assignment, different from its current assignment, it drops its
current link and begins acquisition of its newly assigned link. If
its new assignment is identical to its present assignment, no
change is invoked.
A top level description of the key subprograms of the algorithm,
including the Assign Links procedure, is given in FIGS. 4 through 7
for a three constellation network. FIGS. 5, 6, and 7 set forth in
Ada psuedo code the Determine--Set--of--Candidate--Nodes procedure,
the Determine--Set--of--Candidate--Links procedure, and the
Evaluate--Assignments procedure, respectively from the
Assign--Links procedure of FIG. 4.
When invoked at a node, the Assign Links procedure of FIG. 4 first
checks to see whether a currently active node is isolated from the
rest of the network. If this is the case, all available links at
the isolated node are sent into a coordinated search for
cooperating neighbors. With sufficiently many nodes active, the
global constraint set assigns some nodes which are still a part of
the network to attempt acquisition with the lost node. When the
lost node finds its neighbors, acquisition proceeds normally.
Certain enhancements can be incorporated into this scheme. The
search for unreachable neighbors could be triggered by a diminished
sub-constellation or island size, rather than by complete isolation
of an individual node. This could avoid undesirable modes where
more than one stable island has formed. Also, when a node becomes
unreachable, the distributed topology database still contains
useful information which could guide the recovery process. This
information becomes stale with time, but it could be used for a
limited period to guide the search for neighbors.
Each assignment phase begins by enumerating the set C of candidate
nodes for that phase, as set forth in FIG. 5. For each phase, C is
defined as the set of nodes with at least one transceiver which is
both unassigned and compatible with the class of links which will
be assigned in this phase. For instance, if a particular phase is
reserved for assignment of links between constellations, a node
which has available only a transceiver reserved solely for
intraconstellation links would not be a candidate node and so would
not be a member of set C, but a node with a free transceiver usable
for an interconstellation link would be a member. In addition, the
number of compatible, available transceivers is noted for each
member of set C.
After formation of set C of candidate nodes, the set L is formed
containing all possible pairings of the members of set C, as set
forth in FIG. 6. The algorithm uses several restrictions to limit
the size of this set from becoming too large. First, only node
pairs which are currently in sight of one another need to be
included in the set, because only these pairs could establish a
direct communication link. Second, only node pairs with compatible
transceivers need to be considered, since, for example, a laser
transceiver at one node cannot form a link with an RF transceiver
at another node. Further, it is not necessary to include more pairs
involving a particular node than the number of available
transceivers at that node. To capitalize on this, a subset of the
topology constraints must be formed which can be evaluated by
examining the contribution of individual links. This subset is
transformed into an evaluation function which is used to sort the
possible members of L in order of preference. Then, for any node
with N available links, only the best N pairings are required.
Often more than enough pairings must be retained for some nodes in
order to include sufficiently many pairs for some others. As a
result, not all assignment pairs in L are consistent. For example,
if a node A does not have at least two transceivers available,
selection of a link from node A to node B prevents establishment of
a link from node A to node C.
To determine a final, consistent solution, a graph search procedure
is used to select the best subset of the identified assignment
pairings. This search over the remaining feasible assignment
pairings is conducted by applying constraints 1-4 listed above to
each subset in a prioritized manner. Any topology which meets
constraint 1 is preferred over one which does not. Any topology
which meets constraints 1 and 2 is preferred over one which meets 1
but not 2, etc.
The algorithm applies these constraints globally to all members of
the candidate subset, which satisfy the constraints as to the
entire topology as well as to the individual links. The search
progresses in a "best first" manner, as much as possible, beginning
with those combinations which include the maximum feasible number
of assignment pairings, i.e., half the number of available
transceivers. The best solution among these combinations, if an
acceptable solution is found, is known to be the best overall
solution, since the above constraints require that as many
transceivers as possible be assigned. Therefore, if an acceptable
solution is found after searching these combinations, the search
terminates. Otherwise, the search continues through other
combinations which include fewer and fewer assignments.
If the best solution at this point does not meet the minimum set of
high priority constraints, an existing link is selected whose
termination would liberate transceivers capable of meeting the
mandatory constraints through reassignment. If these high priority
constraints cannot be met, even by breaking and reassigning
existing links, the current assignment phase terminates, returning
the best known assignment set. If a link is selected for breakage,
the current assignment phase is repeated assuming that the
transceivers from that link ar available. After one or more such
iterations, the minimum set of constraints is usually met, and the
current algorithm phase is completed.
The result is a unique solution that is best in terms of the
prioritized list of constraints assigned to the problem and that is
guaranteed to meet designated high priority constraints, if such a
solution exists. When no solution exists which meets all the
required constraints, the algorithm selects the best solution,
meeting as many of the prioritized constraints as possible.
To compute the algorithm, each node 16 must maintain a local copy
of a network topology database in its memory 18 for computing in
its processor 20, as shown in FIG. 8. This can be similar to that
of the New Arpanet routing algorithm. This database has one entry
per node in the network and contains link status and end-point
address for each transceiver at each node The topology database
entry for each node, as defined in Ada pseudo code, is presented in
FIG. 9.
The local copy of this database at each node is updated via the New
Arpanet topology update flood technique. When a transceiver 22
(FIG. 8) enters or leaves the active state, the corresponding node
16 reports this fact to all other nodes by generating a forced
topology update. The term "forced" is used to distinguish it from
normal topology updates generated by the adaptive routing
algorithm. The forced topology update consists of a message
containing the node's address, the current time of day, a sequence
number (incremented for each new topology update) and the node's
LINK--INFO--ARRAY. The node transmits this message over all its
active links. Nodes receiving a topology update compare the
sequence number and time of day in the message to that of the last
received topology update from the sending node as stored in the
memory 18 at each receiving node 16. If the topology update has
already been received, it is discarded. If not, it is copied into
the topology database and forwarded along all active links except
the one on which it was received In this way, all nodes in the
network learn of link failure and acquisition very quickly.
The algorithm is computed by each node when it loses an active link
or receives a forced topology update, but with some minimum
frequency, such as at least once per minute for low earth orbiting
satellites. However, since link failures generally occur in pairs,
i.e. at the two nodes at the ends of a link, it is inefficient to
compute the algorithm immediately after receipt of a forced
topology update. Instead, the computation is delayed by one flood
propagation interval, i.e. the time required for a topology update
flood to traverse the network under worst case conditions, from the
time the forced topology update is received. If any other forced
topology update is received during this time, it is included in the
upcoming computation to insure that all topology updates relating
to a single incident are reflected in the computation. Flood
propagation time is typically less than one second.
Several mechanisms are available for estimating the time that a
pair of nodes in the satellite network will remain in line of sight
of each other. The most accurate of these is direct computation
from orbital dynamics equations, assuming a spherical earth for
simplicity.
In certain constellations, a few satellite pairs may remain in line
of sight of one another constantly due to low relative velocities.
More typically, a given pair of satellites has some maximum time
period T.sub.max over which the satellites can ever remain in line
of sight due to their relative motion. For satellite pairs with a
common finite T.sub.max, the pair of satellites with maximum
expected time in line of sight at time t is the pair that is
nearest together at time ##EQU1## For example, for links from low
earth orbiters to geosynchronous satellites, T.sub.max is
approximately 45 minutes. Therefore, for a given geosynchronous
orbiter, the low earth orbiter which will be in line of sight the
longest at time t is the one that is nearest to that geosynchronous
satellite at time t+22.5 minutes. Thus, a link assignment scheme
requiring maximum expected longevity of the prevailing topology
need only compute link distances at some future instant of time,
rather than iterating through direct line-of-sight calculations
over a time interval. There are some degeneracies in this
approximation, for example when the low earth orbiter's orbital
plane is perpendicular to a line segment connecting the
geostationary satellite with the center of the earth. Nevertheless,
this approximation can be very useful.
For any pair of satellites within a constellation, and thus having
the same altitude, eccentricity, and inclination, the average time
in line of sight over one orbit is a constant. It is a simple
matter to construct a table of these values at each satellite,
however the values are only averages and at any instant in time,
the remaining time in line of sight may differ greatly from these
average values. A selected link with a high estimated time in line
of sight using this approximation may lose line of sight
immediately after it is assigned, or it may continue for twice the
estimated average time. Nevertheless, on the whole, this technique
results in acceptable choices, reducing computational complexity at
the expense of a few poor decisions.
Constraints 1-4 listed above do not include any restriction on the
number of disjoint paths between node pairs. This is a deliberate
reduction in complexity which may result in poor decisions in the
presence of massive node failures. However, it has provided good
results in response to small perturbations, on the order of several
node failures per minute.
Although the present invention has been described with reference to
preferred embodiments, rearrangements and alterations can be made,
and still the result will be within the scope of the invention.
* * * * *